1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus, and more particularly to an image forming method and an image forming apparatus, which form a visual image using a toner.
2. Discussion of the Background
Image forming methods in which an electrostatic image formed on an image bearing member is visualized using a toner are broadly used for various fields. The image forming methods typically include the following processes:    (1) charging an image bearing member (charging process)    (2) irradiating the charged image bearing member with light to form an electrostatic latent image (light irradiating process);    (3) developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member (developing process);    (4) transferring the toner image onto a receiving material optionally via an intermediate transfer medium (transferring process); and    (5) fixing the toner image on the receiving material, resulting in formation of an image (fixing process).
In the fixing process, the toner image on the receiving material is heated and melted by a fixing member such as fixing rollers and fixing belts so as to be fixed on the receiving material.
Recently, in the electrophotographic image forming field, a technology change of from black and white image formation to full color image formation occurs at high speed. Therefore, the full color image formation market is remarkably increasing. In the electrophotographic full color image formation field, a need exists for forming high quality full color images.
On the other hand, a need exists for shortening the warm-up time of electrophotographic image forming apparatuses. Therefore, fixing techniques in that a belt or a film, which has a lower heat capacity than a fixing roller, is used as a fixing member, have been proposed by, for example, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 2002-049258 and 2003-280412. These fixing techniques are practically used.
The fixing members such as belts and films have an advantage in that a wide fixing nip width can be obtained as well as the above-mentioned advantage of shortening the warm-up time, but have a drawback in that a sufficient pressure cannot be applied to toner images. Particularly, when a toner image constituted of a toner having a small particle diameter, which is used to form high quality images, is fixed by such a belt or film fixing member, sufficient energy cannot be applied to the toner. In this case, the toner is insufficiently melted, and therefore the fixed toner image has poor fixing property and low glossiness, which is a fatal flaw for full color images. In this regard, increase in the fixing pressure to improve the fixing property and glossiness causes a problem in that the belt or film used as the fixing member is damaged (i.e., formation of scratches, etc.) after long repeated use, resulting in deterioration of fixing property of toner images.
In order that a fixed toner image has a high glossiness, the toner particles of the toner image have to be fully melted when contacted with a fixing member. Therefore, it is necessary to heat the fixing member to a considerably high temperature. In this case, a hot offset problem in that part of the fixed toner image or the entire toner image is adhered to the fixing member tends to occur. In order to improve the fixing property on the toner side, techniques in that a crystalline polyester resin is included in a toner have been proposed by, for example, JP-A 2003-167384.
In order to prevent the hot offset problem and deterioration of the surface of the fixing member, techniques in that a fixing member whose surface is formed of a material having good releasability from toner (such as silicone rubbers and fluorine-containing resins) is used while applying a liquid having good releasability (such as silicone oils and fluorine containing oils) to the surface of the fixing member have been used.
Although these techniques are effective for preventing the hot offset problem, an applicator for applying an offset preventing liquid to the fixing member has to be provided. Therefore, the fixing device is complicated. In addition, the fixing member easily causes a problem in that one or more layers formed on the fixing member are released from the other layers or the substrate, resulting in shortening of the life of the fixing device.
Therefore, recently oil-less fixing devices without using such an applicator have been proposed.
The toner used for such oil-less fixing devices has to have good releasability from the surface of the fixing member used. For example, the following techniques have been proposed:    (1) a resin having high polymerization degree is used for the toner to increase viscoelasticity of the toner and to impart good releasability to the toner; and    (2) instead of applying a release agent to the fixing member, a toner which includes therein a release agent such as low molecular weight polypropylenes so that the release agent is applied to the surface of the fixing member when the toner is heated is used.
When color images are formed, the fixing member is typically heated to a considerably high temperature. Therefore, it is necessary for the toner mentioned above in paragraph (2) to include a large amount of release agent. In this case, the toner is melted by being heated and pressed by a fixing member, and therefore the release agent in the toner exudes therefrom. The thus exuding release agent is present between the toner and the fixing member, thereby preventing the toner image from adhering to the fixing member, resulting in prevention of occurrence of the offset problem. However, in the above-mentioned fixing devices which use a belt or a film as a fixing member while applying a low pressure to a toner image, the release agent in the toner insufficiently exudes from the toner, and thereby good offset preventing effect cannot be produced.
In addition, when the content of a release agent in a toner is increased, the fixed color toner image has a high haze factor. In this case, the image qualities (particularly, color reproducibility) of the color image deteriorate.
As mentioned above, it is difficult to stably produce color images having good fixing property and high glossiness when a fixing device, which uses a belt or a film as a fixing member while applying a low pressure to the toner image to be fixed, is used.
Because of these reasons, a need exists for an image forming method and apparatus which can stably produce color images having good fixing property and high glossiness by using a fixing device, which uses a belt or a film as a fixing member while applying a low pressure to the toner image to be fixed.